Known methods of processing waste solids such as sewage sludge often involve the use of thermal energy or the production of thermal energy during one or more stages of a treatment process. For example, thermal conditioning may be used in such a manner as to improve the dewaterability of the sludge so that the solid and liquid phases are readily separated, as by sedimentation, filtration, and centrifugation. See, for example: U.S. Pat. No. 3,256,179 issued to Teletzke et al.; U.S. Pat. No. 3,697,417 issued to Teletzke et al.; and U.S. Pat. No. 3,824,186 issued to Erickson et al.
Other methods have been used to obtain heat by the biological decomposition of organic substances and to treat sludge by sterilization. See, for example: U.S. Pat. No. 4,493,770 issued to Moillet, and U.S. Pat. No. 3,959,125 issued to Teletzke.
For economic and environmental reasons, it is desirable to recover and reuse heat produced or used during the treatment of solid waste. Various methods and systems have been proposed to recover such heat energy. Traditional heat recovery systems often used an indirect two step heat transfer process wherein the first step involved transferring heat from treated hot solid waste to water in a first heat exchanger. In a second step, the heat in the water is then transferred to untreated cold waste solids in a second heat exchanger. Such systems are complicated, expensive, and inefficient.
Some heat recovery systems may employ a stainless steel tank placed inside of a larger stainless steel tank wherein each tank acts as a solid waste batch storage tank while heat is being transferred through the wall between the two sludge tanks. The heat is being transferred between the hot solid waste and cold solid waste without the use of an intermediate water media. Often, mechanical mixers are used to agitate the solid waste in each tank to facilitate the heat transfer. These heat recovery systems have very high fabrication costs, are very complex, have high ceiling requirements, and are difficult to maintain and clean. Additionally, the mechanical mixers have the further drawback of becoming clogged over time. As the mixers become clogged or as sludge scale coats the walls of the chambers, it becomes necessary to disassemble and clean the tanks to restore adequate heat transfer efficiency. Disassembly shuts down the whole system.
Spiral heat exchangers have also been used in sludge to sludge heat transfer applications, but present the problems of clogging and high maintenance requirements.